Your DME pick up time will be approximately 3 hours before your flight, so roughly 2:30. You are advised to be at the DME bus stop 15 min prior as the bus can arrive "early", which is 2:15. You need to be back to your resort and ready before this time. I would allow AT LEAST an hour to get back to your resort and then add in whatever time you need to take care of anything before boarding the bus (retrieving carry-ons from bell services, bathroom breaks, etc).
Based on my experience I would say you need to plan on leaving the park by 12:30.